Disclosed are: a compound which is capable of inhibiting the function of HDAC8; and an HDAC8 inhibitor. Specifically disclosed is a hydroxamic acid derivative which is characterized by being composed of a compound represented by general formula (1) (wherein X represents an aromatic substituent or an optionally substituted 3-8 membered ring, and n represents an integer of 0-20), or a pharmaceutically acceptable salt, hydrate, solvate or prodrug thereof.